Spectrum analyzers, which provide a display of the power of the respective frequency components of a signal of interest, can be classified into one of two different types: non-real time and real time instruments. Non-real time instruments, which constitute the vast majority of spectrum analyzers currently in use, customarily employ a sweepable frequency filter or local oscillator which is scanned through the frequency band of the signal being analyzed. If the signal contains a frequency component within the sweep window, the power of that frequency component will be displayed. However, because the sweep window is moving with time, if the signal under analysis has a frequency component occurring outside of the sweep window, the component will be missed and therefore not displayed. Moreover, if the power level of a frequency component is relatively weak, so that it may be masked by noise, it would be necessary to monitor or integrate the frequency component for a period of time that exceeds the dwell time of the sweeping filter window, so that such a frequency component would again be missed.
Real time spectrum analyzers overcome the above shortcomings of swept frequency instruments by simultaneously monitoring the entire signal band of the signal, usually by means of a bank of parallel frequency filters. Current state of the art real time spectrum analyzers employ digital signal processing techniques, which sample and quantize the signal in the time domain. The resulting digitized signal is then processed by means of either a Fast Fourier Transform (FFT) filter or a Finite Impulse Response (FIR) filter to obtain a frequency response output.
The FFT instrument uses the FFT to calculate the frequency domain spectrum from the time domain data. Because of the time required to compute the FFT within size and cost constraints imposed by the instrument market (typically 6.5 ms for a 1024 point real transform), such instruments are limited to a real time bandwidth of less than 200 kHz. The FIR instrument employs a bank of parallel digital FIR filters, which have a limited response time (typically on the order of 200 microseconds per sample), so that a 1024 channel FIR filter instrument also has a relatively narrow real time bandwidth (less than 3 MHz).